Detent escape mechanisms are reputed to be the most precise, and have long been reserved for marine chronometers.
The efficiency of detent escape mechanisms is greater than that of lever escapements, because the escape wheel only communicates an impulse to the balance once per oscillation, during which the escape wheel pivots through one angular step. Consequently, the loss of energy due to the inertia of the escape wheel occurs once per oscillation, as opposed to once per vibration in lever escapements.
The use of detent escapements in wristwatches is more complex, because of the sensitivity of such escapements to shocks.
In the event of shocks, particularly lateral shocks, if the balance is made to pivot outside its normal amplitude, one tooth of the escape wheel may leave the locking pallet stone, and unlocking and impulses occur twice in the same vibration. The effect of this phenomenon, called “tripping”, is to distort the isochronism of the oscillator.
EP Patent No. 1 708, 047 in the name of MONTRES BREGUET SA discloses a lever including an arm carrying both a first finger for cooperating with the second actuating finger, and a feeler with a beak cooperating with a notched cam similar to the preceding ones. When the balance and the plates thereof pivot in a first direction, the first finger drives the second finger to release the locking pallet stone(s) from the escape wheel. The beak of the feeler arm is then driven by a leading flank of the notched cam to re-engage the locking means in the escape wheel. When the balance pivots in the opposite direction, the first finger drives the second finger to keep the locking means engaged in the escape wheel. Whatever the direction in which the plates are pivoting, the encounter between the first and second finger generates a natural rotating force on the lever arbour. This encounter does not generate any risk of breaking the mechanism. No resilient member or stop pins are necessary. In a particular embodiment, this mechanism includes two juxtaposed locking pallet stones including contiguous but not aligned locking faces, which enables the tip of the escape wheel tooth to be housed on a locking line at the junction of the locking faces, creating a draw effect which dispenses with the need for any stop pin. The locking face of the closest locking pallet stone to the escape wheel stands in front of the tooth, and prevents it from continuing on its way. In this total lock position of the escape wheel tooth, the beak of the feeler arm moves away from the periphery of the roller leaving the balance completely free to complete the first vibration. This design makes the escapement resistant to shocks. Indeed, a shock returns the beak onto the periphery of the corresponding roller, without however unlocking the locking pallet stones, since the return of the tooth onto the locking line occurs immediately owing to the draw effect. When, afterwards, during the return movement of the balance in the opposite direction, towards the end of the second vibration, the first finger and the second finger come into cooperation, they give rise to torque in the detent lever around its pivot axis, causing a slight withdrawal of the escape wheel tooth, before, when the fingers separate, the tooth returns to the locking line in an anti-draw effect.
EP Patent No. 1 708 046 in the name of MONTRES BREGUET SA discloses a safety finger secured to the balance roller, and arranged to cooperate with the escape wheel teeth and lock the wheel if the impulse pallet stone should be accidentally released from the toothing of the wheel. This arrangement prevents any disconnection of the escape wheel in the event of a shock resulting in a reversal of the direction of rotation of the plates at the precise moment of the wheel impulse. The encounter of one tooth of the wheel with this security finger locks the wheel and returns the plates to the proper direction of rotation.
EP Patent No 1 522 001 in the name of Detra SA and Patek Philippe SA proposes an escape mechanism with locking parts and toothed wheels with gaps in the toothing. The first wheel set is subjected to a periodic torque, obtained for example by a rotor mounted in a stator. This first wheel set includes, on the one hand, in a basic plane, a first wheel with gaps in the toothing over the periphery thereof, and on the other hand, in a second plane, a first brake-lever including several teeth and able to lock a release lever comprised in a balance roller, when the balance pivots in a first direction. Depending upon its position, this first wheel set cooperates with a second wheel set, either via the first brake-lever or via the first wheel thereof. This second wheel set includes, in the basic plane, a second wheel with toothing gaps, in the second plane, a shaped part which includes several fingers and can lock the balance roller release lever in a second direction of pivoting opposite to the first direction. The second wheel set further includes a locking part in a first plane parallel to the preceding planes. Depending upon its position, this second wheel set cooperates, either via the locking part, or via the second wheel thereof, with an escape wheel, which includes, in the basic plane, a toothed wheel with toothing gaps, and in the first plane, an impulse wheel, which receives a continuous mechanical torque such as that from a barrel, similar to a conventional escape wheel, and can cooperate with the impulse lever comprised in the balance roller, to maintain the oscillating movement of the balance. Depending upon the respective angular positions of the various wheel sets, the locking parts, or shaped parts, or teeth, cooperate with each other, such that the device has four stable locking positions for each revolution of the first pin, between which it has the same number of unlocking positions. The combination of two locking means and two unlocking means for the mechanical torque, and the particular sequence imposing an unlocking operation between two locking operations prevent any racing or tripping in the event of a shock to the mechanism. This mechanism is complex, relatively expensive and extends over several planes, which gives it significant thickness.
EP Patent No. 1 770 452 in the name of Baumberger Peter is an improvement of the Voigt U.S. Pat. No. 180,290 devised to minimise the requirement for space, and it discloses a conventional detent escapement with a detent lever that pivots and is returned by a spiral spring. One arm of the lever carries one end of a strip spring, the other end of which is held abutting on a stop member carried by another arm of the lever, and is arranged to cooperate with a unlocking pallet-stone integral with a small balance roller. Another arm of the lever, beyond a locking pallet stone, includes a finger which cooperates with the periphery of this small roller, and in particular with a truncated portion forming a cam, at a lower level than that of the strip spring. A large balance roller conventionally carries an impulse pallet stone, preceded by a first recess, and followed by a second recess, to allow the locking pallet stone to be unlocked when the unlocking pallet-stone pivots the detent lever. The selection of a particular geometry, both as regards the position of the locking pallet stone and the impulse pallet stone in quasi-symmetry with the line at the centres of the escape wheel and the balance during the locking phase, and the fork formed by the finger and the free end of the strip spring, limit the disruptive effect linked to the detent inertia on the balance oscillations. The amplitude of the pivoting movements of the detent, in the event of shock, is limited by the interaction of the locking pallet stone and the large roller. In a complementary embodiment, this mechanism includes an anti-trip lever, in proximity to the small roller, pivotably mounted on the movement between two stable end positions maintained by a jumper spring on stop members with which a first end can cooperate and the second fork-shaped end of which interacts with the unlocking pallet-stone: each time the unlocking pallet-stone passes into the fork it exerts pressure to tip the anti-trip lever from one stable position to the other. The fork thus forms two stops for the small roller in the event of any tripping, and prevents the balance from pivoting through more than one revolution.
EP Patent No. 1 860 511 in the name of Christophe Claret SA discloses a movement with a moveable bridge, providing protection for a detent escapement against lateral shocks. This moveable bridge carries the sprung-balance pivot axis, the escape wheel pivot axis, the detent pivot axis and part of the gear train. It is pivoted elastically on the arbour of one of the gear train wheels, for example the seconds wheel. Forces, such as a lateral shock, capable of unlocking the locking pallet stone, then drive the entire moveable bridge and the relative positions of the detent and the escape wheel are maintained. This ensures constant operation of the escapement. The moveable bridge may also be dampened by a dampening system which dissipates part of the energy due to the shock.
CH Patent Application No 700 091 in the name of Christophe Claret SA discloses a detent escapement, with a detent lever pivotably mounted on a spiral spring cooperating at the other end with a first strip spring embedded in proximity to the pivot. The balance roller includes two distinct unlocking pallet stones. A wheel set arranged on the opposite side of the escape wheel relative to the detent lever, carries a pivoting cam, holding a cam strip spring and returned towards the detent by a spiral spring on a stop position. This cam is arranged for making the cam strip spring cooperate, either in a first state, with the end of the lever carrying the strip spring, or in a second state, with the unlocking pallet stones of the balance. The first unlocking pallet-stone is arranged to cooperate with the first strip spring and actuate the detent when the first pallet stone encounters the first strip in a first direction, and to cooperate only with the first strip without actuating the detent when it encounters the first strip in the opposite direction. When the cam is in the first state it cooperates with the detent to limit the movements thereof. The second unlocking pallet-stone stone is arranged for changing the cam to the second state in which the detent is free to perform its unlocking operation and release the escape wheel tooth from the locking pallet stone. The two unlocking pallet stones are close and arranged such that the cam is brought into its second state just before the detent performs the unlocking operation. The spiral cam return spring, which is stronger than the cam strip spring, tends to return the cam to its first state. Thus, in the first state thereof, the cam is positioned so as to oppose any inadvertent movement of the detent which could result in inadvertent unlocking of the locking pallet stone, and the escapement is less sensitive to the effects of a shock. Adjustment of this mechanism is complex, since it depends upon the features peculiar to the springs comprised therein, of which there are at least three.
EP Patent No. 2 224 292 in the name of Rolex SA discloses a direct impulse escapement, particularly a detent escapement. The detent lever is arranged in a particular manner, pivoting between two stop members. Facing the escape wheel, it includes a finger including, in succession, a stop surface used as the locking pallet stone, a safety surface which, depending upon the pivotal position of the lever, either interferes or does not interfere with the escape wheel trajectory, and a sliding surface which forces the lever to tip, when the escape wheel is pivoting, so as to return the sliding surface and thus the stop surface to the area of interference with the escape wheel, to stop said wheel pivoting. The balance roller conventionally includes an impulse pallet stone and an unlocking finger. During the vibration in a first pivoting direction of the balance, the lever is in a first stopped pivotal position which allows the unlocking finger to pass, whereas in the other vibration in the other pivoting direction, the lever is pivoted into another stop position and encounters the unlocking finger at an elastic unlocking element comprised in said lever. The elastic travel of this elastic unlocking element allows the balance to continue its travel and the impulse pallet stone passes between two adjacent teeth of the escape wheel. Shortly afterwards, the balance is stopped by the balance spring thereof and pivots in the opposite direction. During this elastic travel, the lever remains butting against the stop member and the stop surface of the lever slides over the escape wheel tooth which is kept stopped. The safety of this mechanism is ensured by the arrangement of the lever finger, with one stop surface and one sliding surface which alternately run into the trajectory of the escape wheel toothing. The length of the safety surface between the stop surface and the sliding surface corresponds to the angle travelled by the wheel to communicate the drive energy to the balance, to prevent the premature return of the stop element into the trajectory of the wheel, which provides additional security. Part of the energy from the barrel is, however, consumed in friction during the sliding phase.
Few Patent documents are specifically dedicated to anti-trip mechanisms.
EP Patent No. 1 645 918 in the name of MONTRES BREGUET SA discloses a mechanism of this type, including a finger secured to an arm of the balance. The balance cock includes two columns, between which this finger can pass. A locking arm is secured to the outer coil of the balance spring, particularly by clamping, and when the balance tends to race under the effect of a shock and exceed its normal amplitude, the arm can abut on the columns to prevent the finger from passing.
EP Patent No. 1 801 668 in the name of MONTRES BREGUET SA discloses an arrangement of a balance axis fitted with a pinion, which cooperates with a toothed sector that can move between two stop positions and prevents the balance from pivoting beyond its normal amplitude.
In short, known embodiments are often complex and difficult to adapt from one escape mechanism to another. The method of stopping the balance is generally very abrupt and does not always ensure that the movement will restart without intervention.